The invention relates to a locking device comprising a cylinder lock and reversible flat key, having indentations located on opposite broad faces of the key for the arrangement of tumbler pins, and having a rib associated with each row of indentations, into the sectional area of which at least one of the pins extends, and the crest of which rib projects beyond the broad face of the key. A locking device of this kind is described in DE-PS No. 2 658 495, in which the locking indentations, departing from the crest of the rib, are incorporated into the reversible flat key. The ribs are projecting bulges of material, compared with the normal profile. The total thickness in the region of the ribs cannot however be used for the provision of indentations of maximum depth, since otherwise the indentations lying on the same cross-sectional plane would penetrate each other. In practice, the locking indentations are usually produced by a boring cutter having a diameter of approximately 4.5 mm, which is of course greater than that of the tumbler pins. If a boring cutter were to be used, the diameter of which was close to that of the tumbler pins, then for an indentation, the height of which is greater than that of the tumbler pin point, insertion or withdrawal of the reversible flat key could be made impossible because of the catching step formed along the length of the key.